21086815 has 4 divisors (see below), whose sum is σ = 25304184. Its totient is φ = 16869448.

The previous prime is 21086753. The next prime is 21086839. The reversal of 21086815 is 51868012.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21086815 - 2⁷ = 21086687 is a prime.

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2108677 + ... + 2108686.

It is an arithmetic number, because the mean of its divisors is an integer number (6326046).

Almost surely, 2^21086815 is an apocalyptic number.

21086815 is a deficient number, since it is larger than the sum of its proper divisors (4217369).

21086815 is an equidigital number, since it uses as much as digits as its factorization.

21086815 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4217368.

The product of its (nonzero) digits is 3840, while the sum is 31.

The square root of 21086815 is about 4592.0382184821. The cubic root of 21086815 is about 276.2720790690.

It can be divided in two parts, 210 and 86815, that added together give a square (87025 = 295²).

The spelling of 21086815 in words is "twenty-one million, eighty-six thousand, eight hundred fifteen".